In general, starting of a fuel cell power system is carried out by firstly heating a fuel cell stack to a temperature at which fuel cell reactions may take place, feeding process gases to the fuel cell stack, and then loading a direct current resistance to the output circuit of the fuel cell stack to heat the same with heat of fuel cell reactions to a predetermined operating temperature, or, its optimum operating temperature. At the predetermined operating temperature, the fuel cell stack generates its rating output power which is supplied to an external load. In pressurized fuel cell power systems of the kind, it is required to pressurize the process gases, cooling gas, and inert gas filled in the pressure vessel to the rating pressure after the fuel cell stack is heated to the predetermined temperature.
However, it is difficult with the conventional method of starting to properly control the operating conditions of the fuel cell stack during its starting process, since there are many variable parameters to be controlled such as, for example, a temperature of the fuel cell stack, pressures and supplies of the process gases, a vapor pressure in the process gases, a discharging current, and the like. If any mismatch of the variable parameters occurs during starting operation, the stack would be exposed to severe conditions, resulting in lowering in the life characteristics of the power system.
In particular, if a hygroscopic material, for example, phosphoric acid is used as the electrolyte, there may occur considerable variation in the volume of electrolyte in proportion to a vapor pressure in the process gases. If the volume of phosphoric acid electrolyte is increased considerably by absorption of water, the electrolyte would be leaked into gas diffusion layers (backing paper) and leaked out from the fuel cell stack, resulting in lowering of the characteristics. If the volume of the electrolyte is considerably decreased by evaporation, there may occur considerable increase in cell resistance and/or crossover of the process gases, resulting in deterioration of the characteristics. Accordingly it is very important to control the volume of the electrolyte to maintain a good performance of the fuel cell power system for a long period of time.